As the development of nanoscale mechanical, electrical, chemical and biological devices and systems increases, new processes and materials are needed to fabricate nanoscale devices and components. Optical lithographic processing methods are not able to accommodate fabrication of structures and features at the nanometer level. The use of self-assembling diblock copolymers presents another route to patterning at nanometer dimensions. Diblock copolymer films spontaneously assembly into periodic structures by microphase separation of the constituent polymer blocks after annealing, for example by thermal annealing above the glass transition temperature of the polymer or by solvent annealing, forming ordered domains at nanometer-scale dimensions. Following self assembly, one block of the copolymer can be selectively removed and the remaining patterned film used as an etch mask for patterning nanosized features into the underlying substrate. Since the domain sizes and periods (Lo) involved in this method are determined by the chain length of a block copolymer (MW), resolution can exceed other techniques such as conventional photolithography, while the cost of the technique is far less than electron beam (E-beam) lithography or EUV photolithography, which have comparable resolution.
The film morphology, including the size and shape of the microphase-separated domains, can be controlled by the molecular weight and volume fraction of the AB blocks of a diblock copolymer to produce lamellar, cylindrical, or spherical morphologies, among others. For example, for volume fractions at ratios greater than about 80:20 of the two blocks (AB) of a diblock polymer, a block copolymer film will microphase separate and self-assemble into a periodic spherical domains with spheres of polymer B surrounded by a matrix of polymer A. For ratios of the two blocks between about 60:40 and 80:20, the diblock copolymer assembles into a periodic hexagonal close-packed or honeycomb array of cylinders of polymer B within a matrix of polymer A. For ratios between about 50:50 and 60:40, lamellar domains or alternating stripes of the blocks are formed. Domain size typically ranges from 5-50 nm.
Periodic cylindrical structures have been grown in parallel and perpendicular orientations to substrates. A primary requirement for producing perpendicular cylinders by thermal annealing is that the substrate floor must be neutral wetting to the blocks of the copolymer. Periodic hexagonal close-packed cylinders can be useful as etch masks to make structures in an underlying substrate for applications such as magnetic storage devices. However, that layout is not useful for making structures such as DRAM capacitors, which require a rectangular- or square-shaped array layout.
Graphoepitaxy techniques using substrate topography have been used in an attempt to influence the orientation, ordering and registration of the microphase-separated domains. Although one-dimensional arrays have been formed in trenches, no efforts have been made to address ordering of the domains over a large area, or to control the location and orientation of ordered domains in two dimensions.
Although there is a single report of forming ordered sphere-forming block copolymer films by Cheng et al. (Nano Lett., 6 (9), 2099-2103 (2006)), these have been limited to one-dimensional ordered arrays with adjacent arrays not aligned, the cylinders being off-set along the y-axis in neighboring trenches.
It would be useful to provide methods of fabricating films of two-dimensional arrays of ordered nanostructures that overcome these problems.